“Revealing the processes by which organic carbon compounds form on Mars has been a matter of tremendous interest for understanding its potential for habitability,” said Carnegie’s Andrew Steele. “The discovery that natural systems can essentially form a small corrosion-powered battery that drives electrochemical reactions between minerals and surrounding liquid has major implications for the astrobiology field.” A similar process could occur anywhere that igneous rocks are surrounded by brines, including the subsurface oceans of Jupiter’s moon Europa, Saturn’s moon Enceladus, and even some environments here on Earth, particularly early in this planets’ history.

Carnegie’s Andrew Steele was a key member of the research team whose work on this project built off his discovery six years ago of indigenous organic carbon in 10 Martian meteorites. The organic molecules he found in 2012 are comparable to those found by Curiosity.

Like the meteoric samples, the rocks sampled by Curiosity must be heated by the rover’s instruments to very high temperatures, ranging between 500 and 800 degrees Celsius (932 and 1,472 degrees Fahrenheit), to have their organics released as gas. Because the hydrocarbons were released at such high temperatures, they may be coming from bigger, tough organic molecules within the rock.

Sedimentary rocks (mudstones) were drilled from four areas at the base of Mount Sharp, the central mound in Gale crater. Although the surface of Mars is inhospitable today, there is evidence that in the distant past, the Martian climate allowed the presence of liquid water–an essential ingredient for life–at the surface.

Analysis by Curiosity indicates that billions of years ago, a lake inside Gale crater held all the ingredients necessary for life, including chemical building blocks, energy sources, and liquid water. The mudstone gradually formed from silt that settled out of the water and accumulated at the bottom of the lake. Scientists estimated the age of the rocks by the crater count method. Since meteorite impact craters accumulate over time, the more craters a region has, the older it is. Although there was no way to directly date the organic material found within the rocks, it has to be at least as old as the rocks themselves.

The results indicate organic carbon concentrations on the order of 10 parts per million or more. This is close to the amount of observed in Martian meteorites and about 100 times greater than prior in-situ detections of organic carbon. Some of the molecules identified include thiophenes, benzene, toluene, and small carbon chains, such as propane or butene. Organic molecules containing chlorine were detected on Mars before.

Finding ancient carbon preserved right on the Martian surface gives scientists confidence that NASA’s Mars 2020 rover and the European Space Agency’s ExoMars rover will find even more organics, both on the surface and in the shallow subsurface.

“Are there signs of life on Mars?” asks Michael Meyer, NASA Program Scientist for the Mars Science Laboratory mission. “We don’t know but these results tell us we are on the right track.”

Steele says that the next steps must be looking for organic compounds that are released from the rock samples at lower temperatures.

Mars’ organic carbon may have originated from a series of electrochemical reactions between briny liquids and volcanic minerals, according to new analyses of three Martian meteorites from a team led by Carnegie’s Andrew Steele. The group’s analysis of a trio of Martian meteorites that fell to Earth—Tissint, Nakhla, and NWA 1950—showed that they contain an inventory of organic carbon that is remarkably consistent with the organic carbon compounds detected by the Mars Science Laboratory’s rover missions.

Steele’s 2012 team determined the organic carbon found in 10 Martian meteorites did indeed come from the Red Planet and was not due to contamination from Earth, but also that the organic carbon did not have a biological origin. This new work takes his research to the next step—trying to understand how Mars’ organic carbon was synthesized, if not by biology.

High-resolution Transmission Electron Micrograph (scale 50nm) of a grain from a Martian meteorite. Reminiscent of a long dinner fork, the organic carbon layers are found between the intact ‘tines.’ This texture is created when the volcanic minerals of the Martian rock interact with a salty brine and become the anode and cathode of a naturally occurring battery in a corrosion reaction. This reaction would then have enough energy–under certain conditions–to synthesize organic carbon. Credit: Andrew Steele

Organic molecules contain carbon and hydrogen, and sometimes include oxygen, nitrogen, sulfur, and other elements. Organic compounds are commonly associated with life, although they can be created by non-biological processes as well, which are referred to as abiotic organic chemistry.

He and his co-authors took a deep dive into the minerology of these three Martian meteorites. Using advanced microscopy and spectroscopy, they were able to determine that the meteorites’ organic compounds were likely created by electrochemical corrosion of minerals in Martian rocks by a surrounding salty liquid brine.